Printer carrier drive

ABSTRACT

A new and improved technique and device is disclosed for selectively coupling a print wheel carrier with a constantly rotating lead screw. The engaging device comprises a rotatable engaging member which is in rolling engagement with the lead screw. This relationship reduces the friction and wear between the members. The technique also facilitates rapid decoupling and minimizes carrier over-travel.

United States Patent [191 Sweeney [451 Sept. 11, 1973 PRINTER CARRIERDRIVE [75] Inventor: David F. Sweeney, Pittsford, N.Y.

[73] Assignee: The Singer Company, New York,

[22] Filed: Aug. 30, 1971 [21] Appl. No.: 176,046

[52] U.S. Cl. 197/49, 101/93 C [51] Int. Cl B4lj 1/22 [58] Field ofSearch 74/57; 197/49; 101/93 C [56] References Cited UNITED STATESPATENTS 1,177,609 4/1916 Edwards 74/57 2,330,082 9/1943 Side et a1 74/572,843,243 7/1958 Masterson.... 197/49 X 3,049,803 8/1962 Danger 74/57 X3,059,482 10/1962 Petz01dt.... 74/57 X 3,080,765 3/ 1963 Eisele 74/573,406,625 10/1968 Chamness et 74/57 X 3,424,291 1] 1969 Marion 197/493,651,914 3/1972 Locke 197/49 Primary Examiner-Edgar S. BurrAttorney-George W. Killian et a1.

[5 7 ABSTRACT A new and improved technique and device is disclosed forselectively coupling a print wheel carrier with a constantly rotatinglead screw. The engaging device comprises a rotatable engaging memberwhich is in rolling engagement with the lead screw. This relationshipreduces the friction and wear between the members. The technique alsofacilitates rapid decoupling and minimizes carrier over-travel.

9 Claims, 7 Drawing Figures Patented Sept. 1 1, 1973 4 Sheets-Sheet 1INVENTOR. DAVID F. SWEENEY /awaz. w. 1%

AGENT Patented Sept. 11, 1973 4 Sheets-Sheet 2 INVENTOR. DAVID F.SWEENEY AGENT Patented Sept. 11, 1973 3,757,922

4 Sheets-Sheet FIG.3

INVENTOR.

' DAVID F. SWEENEY AGENT Patented Sept. 11, 1973 7 3,757,922

4 Sheets-Sheet 4,

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FIG.4

:02 Pnunnunnnu INVENTOR. DAVID F. SWEENEY AGENT PRINTER CARRIER DRIVECROSS REFERENCE TO RELATED APPLICATIONS The invention disclosed hereinis suitable for use in a printing mechanism which may also incorporatethe inventions disclosed and described in the co-pending applications ofDonald G. Hebert, filed Dec. 7, 1970, assigned Ser. No. 95,645, andentitled BALLISTIC PRINT HAMMER MECHANISM and also the copendingapplication of Donald G. Hebert and Richard G. Dobner filed May I2,1971, assigned Ser. No. 142,702, and entitled PRINTER PAPER GUIDE WHICHCOMPENSATES FOR THICKNESS OF DOCUMENT PACK. Both of these co-pendingapplications are assigned to the same assignee as the presentapplication.

FIELD OF THE INVENTION This invention relates to a high speed on-the-flyprinting mechanism and more particularly to a character-by-characterprinter employing a rotatable print wheel with alphanumeric charactersarranged in a helical pattern. Even more particularly, the inventionrelates to a means whereby a carrier for supporting the rotatable printwheel may be selectively engaged with a constantly rotating lead screwfor moving the carrier and the rotatable print wheel in a path of travelwhich is parallel to the axis of rotation of the print wheel.

DESCRIPTION OF' PRIOR ART US. Pat. Nos. 3,442,364, issued May 6, 1969,to Robert A. Ragen; 3,406,625, issued Oct. 22, 1968, to L. D. Chamnesset al.; and 3,424,291, issued .Ian. 28, 1969, to A. F. Marion, discloseprinters of the general character described. However, the cited patentsdo not disclose a carrier which may be selectively coupled with thedriving means at any point intermediate of the ends of the printingline. The Chamness et al. patent does disclose a pin 58 which is urgedinto an engagement with the lead screw for moving the carrier in theprinting direction. The Chamness et al. pin is subjected to slidingfriction with the lead screw and therefore, may experience wear and alarge force may be necessary to withdraw it from engagement with thelead screw.

SUMMARY OF THE INVENTION The present invention discloses a new andimproved means for selectively coupling a carrier to a constantlyrotating leadscrew. The coupling means comprises a rotatable engagingdisc designed to mate with the lead screw threads and means for urgingthe rotatable engaging disc towards the lead screw only when theinstantaneous angular position of the lead screw is such that therotatable disc will engage with the threads of the lead screw with anon-interfering fit. The engaging disc rotates on its axis during thetime that it is in engagement with the lead screw and therefore, thefriction between the engaging member and the lead screw is greatlyreduced over that of the prior art which em' ployed sliding members. Thereduced friction means reduced wear. In addition, because of the reducedfriction between the engaging disc and the lead screw, it is easier towithdraw the engaging disc from the lead screw when it is desired toeffect a disengagement.

E ation, the retaining pawl is electromagnetically withdrawn from therack and released immediately after the carrier commences its returnmotion. Accordingly, the retaining pawl will return to a retainingposition and stop the carrier when the carrier moves sufficiently far inthe return direction for the pawl to engage the next tooth of the rack.With such a backspacing system, it is important that the carrier doesnot experience significant over-travel. That is, if the carrier shouldexperience over-travel, there is a possibility that a backspace signalcould be received, the retaining pawl withdrawn from the rack andreleased, all in less time than it would have taken the carrier to fallback the amount of the over-travel. Thus, under such circumstances,there would have been a failure of the carrier to achieve an actualbackspacing operation. The present structure provides a means forassuring that there is minimum over-travel of the carrier and therebyassures that there will be proper backspacing action in response to eachbackspacing signal.

It is an object of this invention to provide a'new and improved meansfor selectively coupling a print wheel carrier to a constantly rotatinglead screw.

It is a more particular object of this invention to provide a couplingmeans of the character described which is subject to reduce wear.

It is another object of the invention to provide a new and improvedmeans for coupling a print wheel carrier to a lead screw wherein thecoupling means comprises a rotatable disc. I

It is another object of the invention to provide a new and improvedprint wheel carrier coupling means which has reduced frictional forcesduring engagement.

It is another object of the invention to provide a print wheel carriercoupling means for selective engagement and disengagement with aconstantly rotating lead screw.

It is another object of the inventionto provide a new and improved printwheel carrier means which may be disengaged from a lead screw with aminimum overtravel of the carrier.

It is another object of the invention to provide in combination with anew and improved print wheel carrier coupling means a positive actionsingle pawl backspacing mechanism.

BRIEF DESCRIPTION OF THE DRAWING The invention both as to itsorganization and method of operation will best be understood byreference to the following detailed description taken in connection withthe accompanying drawing in which like numerals always refer to likeparts and in which:

FIG. 1 comprises a side view of a printer mechanism incorporating theinvention;

FIG. 2 comprises a more detailed view of the invention and moreparticularly a view showing the engaging disc out of engagement with therotating lead screw;

FIG. 3 is similar to FIG. 2 but shows the engaging disc in engagementwith the rotating lead screw;

FIG. 4 shows an end view of the engaging disc and the supportingstructure;

FIG. 5 shows the backspacing mechanism; and

FIGS. 6 and 7 show a timing mechanism for determining the instantaneousangular position of the lead screw and/or of the print wheel.

DESCRIPTION OF THE PREFERRED EMBODIMENT carrier frame casting 102 isslideably supported on the detent and support rail 103. As the namesuggests the detent and support rail 103 includes a detent rack 104which, as will be seen, cooperates with a detent pawl, to be describedlater, to retain the print wheel carrier 101 at any position along thelength of the detent and support rail 103. The print wheel carrier 101is additionally supported by the main carrier guide rail 105. Coupled tothe carrier frame casting 102 are guide shoes 106 and 107 whichslideably engage the main carrier guide rail 105. The position of theguide shoes 106 and 107 relative to the carrier frame casting 102 may beadjusted by screws 108 and 109 to thereby provide some control over thepositioning of the print wheel carrier 101. The detent and support rail103 and the main carrier guide rail 105 are supported by end framemembers 110.

A printshaft 112 is also supported by the machine end frame members 110and journaled for rotation. The print shaft 1 12, the detent and supportrail 103 and the main carrier guide rail 105 are substantially parallel.A lead screw 113 is also supported by the machine end frame members 110and journaled for rotation. The print shaft 112 and the lead screw 1 13are coupled together by gears, or cog belts, so that there is apredetermined relationship relative to their angular rotation. In oneembodiment of the printer the print shaft 112 makes two revolutions foreach revolution of the lead screw 113. Any suitable and conventionalmeans may be provided for selectively rotating the print shaft 112,

and the lead screw 113, at a predetermined angular velocity. It shouldbe noted that the print shaft 112 has key slots 114. Mounted on theprint shaft 112 is a print wheel assembly 118 which comprises a unitaryhub 1 19 and a flange 120. Between the print shaft 112 and the hub 119is a sleeve bearing 121. A print wheel 122 is secured to the flange 120by means of screws 123. As seen in the view of FIG. 1 the print wheel122 is on the far side of the flange 120 and has an inner diameter asshown by the dotted line 124. In addition'to the print wheel [22, whichis coupled to the flange 120, there is coupled thereto a pair of keys(not shown) which engage the key slots 114. The coupling keys are notshown as they do not form a significant feature of this invention andwould only tend to obscure the drawing. However, it should be understoodthat the print wheel assembly 118 is keyed to the print shaft 112 andthat, therefore, the print wheel assembly 118 will rotate with the sameangular velocity as the rotation of the print shaft 112. Furthermore,although the print wheel assembly 118 is keyed to the print shaft 112 itshould be understood that the print wheel assembly 118 will slide alongthe length of the print shaft 112. The sleeve bear- I ing 121 isprovided to reduce the friction and wear as the print wheel assembly 118slides longitudinally on the print shaft 112.

As will be seen the print wheel carrier 101 slides longitudinally on thedetent and support rail 103 and is guided and kept from rotationalmotion by the sliding relationship between guide shoes 106 and 107 withthe main carrier guide rail 105. Arm 128 of carrier frame casting 102supports a print wheel drive plate 129 which has a portion 130 whichextends into a slot 131 in hub 119. The width of the print wheel driveplate portion 130 and the width of the slot 119 are substantially equal.Accordingly, as the print wheel carrier 101 is moved longitudinally ofthe detent and support rail 103 the print wheel assembly 118 willundergo corresponding longitudinal motion. That is, when there islongitudinal motion of the print wheel carrier 101 the print wheel driveplate portion 130 which engages slot 131 of hub 119 causes longitudinalmovement of the print wheel assembly 118.

It should be understood that the print wheel 122 is a disc and that onthe outer surface of its periphery will be found alphanumericcharacters. The print wheel 122 and the print hammer assembly 134(described more fully-in the cited Hebert application) will cooperate toprint selected alphanumeric characters on a document (not shown) locatedbetween the print wheel 122 and the print hammer assembly 134. Ink isapplied to the alphanumeric characters of the print wheel 122 by meansof an ink cartridge 136. The ink cartridge 136 houses an ink roller (notseen) which is rotationally supported within the ink cartridge 136. Theink cartridge 136 is pivotally supported at pivot support 137 and isbiased towards the print wheel 122 by spring 138 which has one endcoupled to the ink cartridge 136 and the other end coupled to arm 139which is fastened to a part of the carrier frame casting 102 by screws140.

The print wheel carrier 101 is moved in the print direction by couplingthe print wheel carrier 101 to the constantly rotating lead screws 113by the letter space roller 151. That is, the lead screw 113 has ahelical thread and when the letter space roller 151 is engaged with thethreads of the lead screw 113 the print wheel carrier 101 will be movedlongitudinally in the letter spacing direction. The letter space roller151 is rotationally mounted on a roller shaft 152 (FIG. 4) which issupported in roller housing 153 by roller shaft yoke 154 (best seen inFIG. 4) and having a yoke tail portion 155 which extends to the left ofthe roller housing 153 as seen in FIG. 1. Spring 156 biases the rollershaft yoke 154, and hence the letter space roller 151, away from, andout of engagement with, the lead screw 1 13. When it is desired toengage the letter space roller 151 with the lead screw 113 the letterspace solenoid 161 is electrically energized and it drives the rollershaft yoke 154, and hence the letter space roller 151, to the right asseen in FIG. 1. The letter space solenoid 161 is mounted on the carrierframe casting 102 by mounting loop 162 and a pair of screws 163, onlyone of which is seen in FIG. 1.

Considering now more specifically FIG. 2 wherein there is shown anenlarged and more detailed view of the elements associated with theletter space roller 151 it will be seen that the roller housing 153 issecurely but adjustably positioned on the carrier frame casting 102 byscrews 164. Back stop screw 165 limits the leftward motion of thesolenoid plunger 166 and thereby determines the maximum leftwardposition of the roller shaft yoke 154 and accordingly determines theclearance between the letter space roller 151 and the teeth 167 of thelead screw 113.

Consideration should now be given to FIG. 3 which is similar to FIG. 2except that a portion of the roller housing 153 has been cut away tomore clearly show the roller shaft yoke 154 and the letter space roller151. In addition, in FIG. 3 the letter space solenoid 161 is operatedand therefore it has urged the letter space roller towards the leadscrew 113 so that the letter space roller 151 will engage the threads167 of the lead screw 113. The letter space roller 151 is designed tomate with the threads 167 of the lead screw 113 and so that the letterspace roller 151 touches the sides of adjacent teeth 167 of lead screw113 but not the root of the teeth. As may be seen by comparing FIGS. 1and 3 the roller shaft 152 is supported by the roller shaft yoke 154 andguided by slots 157 in the roller housing 153.

FIG. 4 is a view taken along line 44 of FIG. 2. As may be seen fromFIGS. 3 and 4 the letter space roller 151 is centered between the arms158 of the roller shaft yoke 154 by the roller bosses 168. It should beparticularly noted from FIG. 4 that the roller shaft 152 is nothorizontal. That is, the roller shaft 152 is tilted from the horizontala few degrees so that the letter space roller 151 can mate properly withthe helical teeth 167 of the lead screw 113. Or phrased differently theaxis of the roller shaft 152 is at right angles to the angle of lead ofthe helical thread 167 of the lead screw 113. Furthermore, when theletter space roller-151 is engaged with the teeth 167 and the lead screwis rotated there will be a rolling contact between the letter spaceroller 151 and the teeth 167. v

FIG. 5 illustrates the backspace and carrier return so-' lenoid 176which is located on the print wheel carrier I01 and supported on theunderside of the carrier frame casting 102. These elements are not shownin FIG. 1 as they would not be seen clearly and would only tend toobscure FIG. 1. A solenoid base plate 177 is attached to the carrierframe casting 102 by screws 178 and the backspace and carrier returnsolenoid 176 is attached to the solenoid base plate 177 by a mountingloop 179 whichiis secured to the solenoid base plate 177 be screws 180.In response to the electrical operation of the backspace and-carrierreturn solenoid 176 the plunger 181 is moved from right to left asviewed in FIG. 5. Coupled to the plunger .181 is a detent pawl linkage182 which is also coupled at its other end to detent pawl 183. Thedetent pawl 183 includes a pawl dog 184 which, when the back-space andcarrier return solenoid 176 is not energized, will engage the teeth ofdetent rack 104 on the detent and support rail 103. The engagement ofthe pawl dog 184 with the teeth of detent rack 104 will inhibit thelongitudinal motion of the print wheel carrier 10] in a carrier returndirection. That is, the print wheelcarrier 101 is biased towards acarrier return direction by a bias force indicated schematically at 185.Accordingly, when it is desired to initiate a carrier return actuationthe backspace and carrier return solenoid 176 may be actuated to therebypivot the detent pawl 183 about its pivot point 186 and withdraw thedetent pawl dog 184 away from engagement with the teeth of detent rack104. Spring 187 is coupled to the solenoid base plate 177 and the detentpawl 183 to bias the detent pawl 183 so that the pawl dog 184 willengage the teeth of the detent rack 104.

The detent pawl linkage 182 comprises two identical parts with one oneach side of the detent pawl 183 and coupled to the detent pawl 183 by apin 188 which passes through a hole 189 in the detent pawl 183. Itshould be noted that the hole 189 is oversize with respect to the pin188. Accordingly, as the print wheel carrier 101 moves in a letterspacing direction the detent pawl 183 is enabled to pivot about pivotpoint 186 and ride over the teeth of the detent rack 104 withoutimparting a motion to the detent pawl linkage 182 and the plunger 181.

The various electrical connections that must be made to the electricalcomponents on the print wheel carrier 101 are made through a flexibleribbon 190 which carries a plurality of electrical conductors.

FIG. 6 shows an end view of the print shaft 112 and a timing wheel 195which is attached to the print shaft 112. Towards the outer perimeter ofthe timing wheel 195 are located a plurality of timing slots 196 withone slot 196 for each character located on the periphery of the printwheel 122. In addition, there is a wide timing slot 197 whichcorresponds to the space between the last and first character on theprintwheel 122 and provides a reset signal for the associated equipment.The spaces on the timing wheel 195 between the timing slots 196 areopaque.

FIG. 7 shows a side view of the timing wheel 195 and indicates that alight source 198 can shine through the timing slots 196 and/or the widetiming slot 197 and be detected by the light detector 199 to provide asignal indicative of the instantaneous position of the print wheel 122.By standard techniques, which are well known to those skilled in therelated arts, counters may be used together with comparison circuits forproviding a signal to operate the print hammer assemblyv 134 to affectthe printing of a desired character. In an actual application the lightsource 198 may comprise a light emitting diode. v

The timing wheel 195 and associated equipment provide another signalwhich is very important to the effective and efficient utilization ofthe invention. More specifically, the structure of the present inventionis designed to permit the selective coupling of the print wheel carrier101 with. the constantly rotating lead screw 113. When the print wheelcarrier 101 is not engaged with the lead screw 113 the detent pawl 183retains the print wheel carrier 101 at its attained position. When it isdesired to engage the print wheel carrier 101 with the lead screw 113the letter spacesolenoid 161 is energized to move the letter spaceroller 151 into engagement with the teeth 167 ofthe lead screw 113.-Ob-

' viously, such engagement between the roller 151 and the teeth 167 canonly be affected when the angular position of the lead screw 113 is suchthat a valley between two of the teeth 167 is presented to the roller151. That is, if an attempt were made to move the roller 151 towards thelead screw 113 at any other time there would be an interference andpossible damage. It will be recalled that the print shaft 112 and thelead screw 113 are coupled together. Accordingly, if the angularmovement of one relative to the other is known it is possible todetermine the angular position of one if the angular position of theother is known. The timing wheel 195, and its associated equipment,always provides information relative to the instantaneous angularposition of the print shaft 112. Accordingly, with the angualr positionof the print shaft 1 12 known the angular position of the lead screw 113is also known. Accordingly, a signal derived from the timing wheel 195and a selected one of the timing slots 196 may be used to indicate aappropriate time for operating the letter space solenoid 161 forengaging the letter space roller 151 with the lead screw 113.

In one embodiment of a machine incorporating the present invention thelead screw 113 makes only half a revolution for each revolution of the'print shaft 112. However, the lead screw 113 is double threaded. Thatis, there are two parallel spiral threads on the lead screw 113 so thatalternate teeth 167 form one thread and the remaining teeth 167 form theother thread. Accordingly, once each half revolution of the lead screw113 a valley between adjacent teeth 167 is presented to the roller 151and the letter space solindoid 161 may be actuated to engage the roller151 with the lead screw 113. Or, phrased differently, once perrevolution of the print wheel 122 there is an opportunity to engage theprint wheel carrier 101 with the lead screw 113.

It should be'understood that the characters on the print wheel 122 arearranged thereon in a helix. That is, when the print wheel 122 isengaged with the lead screw 113 the print wheel'carrier 101 will bemoved a small amount in the letter space direction for each increment ofrotation of the lead screw 113. More specifically, the print wheel 122will be moved one letter space width in the letter space direction foreach complete revolution of the print wheel 122. Phrased differently,adjacent teeth 167 on the lead screw 113 are spaced apart by an amountequal to the letter space distance; usually 0.1 inch. To compensate forthe constant motion of the print wheel 122 in the letter spacingdirection, and to be enabled to print each character on the print wheel122 within a predetermined letter space position, the successivealphanumeric characters on the print wheel 122 do not have their centerlines in a common plane which is at right angles to the axis of the leadscrew 113. That is, each successive one of the alphanumeric charactershas its center in a plane which is parallel to, but removed from, thecorresponding plane of adjacent characters by an amount whichcorresponds to the longitudinal motion of the print wheel 122 thatresults from the angular rotation of the print wheel 122 as the printwheel 122 rotates the number of degrees necessary to bring successivecharacters to the printing point. Accordingly, every one of thealphanumeric characters on the print wheel 122 will be presented withinthe limits of each letter space position on the document. Charactersdisplaced on a print wheel in this manner are sometimes said to beplaced in a helical pattern and therefore printers having thischaracteristic are sometimes referred to as helical printers.

It should be observed that the roller 151 is rotatably mounted on theroller shaft 152 and that while the roller 151 is in contact with theteeth 167 of the lead screw 113 there is rolling contact between theroller I51 and the sides of the teeth-167. Accordingly, the wear betweenthe roller and the teeth 167 is greatly reduced over what it would be ifthere were a sliding contact. The roller 151 is maintained in contactwith the threads 167 of the lead screw 113 by the force applied by theletter space solenoid 161 acting through the roller shaft yoke 154. Assoon as the letter space solenoid 161 is de-energized the continuedrotation of the lead screw 113 will apply a force to the roller 151 toeject the roller 151 from contact with the lead screw 113. In addition,the spring 156 provides a force to extract the roller 151 from itscontact with the lead screw 113. Accordingly, when it is desired toterminate the coupling between the print wheel carrier 101 and the leadscrew 113 such termination may be affected quite rapidly. It should benoted that in a preferred embodiment of the invention the roller 151 isnot extracted from the lead screw 113 at an indiscriminate time. Morespecifically, the extraction of the roller 151 from the threads 167 ofthe lead screw 113 is affected in response to the receipt of a signalfrom a predetermined slot 196 of the timing wheel 195 such that therehas been enough travel of the print wheel carrier 101 in the letterspace direction to assure that the detent pawl dog 184 will engage thenext tooth on the detent rack 104. However, as will be seen, a minimumamount of overtravel is desired.

In a preferred embodiment of the machine the detent pawl 183 is used toinitiate a backspacing actuation. Consider first the backspacingoperation in response to a signal which occurs at a time while theroller 151 is not engaged with the lead screw 113. In response to thereceipt of the backspacingsignal the backspace and carrier returnsolenoid 176 may be actuated by any convenient means familiar to thoseacquainted with such mechanisms. In response to the actuation of thebackspace solenoid 176 the detent pawl 183 will be pivoted away from thedetent rack 104 and the bias force indicated at will start to move theprint wheel carrier 101 in a backspacing direction. Before the printwheel carrier 101 has had time to move as much as one letter space in abackspacing direction the backspace and carrier return solenoid 176 willbe released and the detent pawl 183 will pivot about pivot point 186 inresponse to the force of spring 187 and caused the detent pawl dog 184to engage the very next tooth of the detent rack 104 thereby limitingthe motion of the carrier -l01 to one letter space.

Consider now a situation wherein the print wheel carrier 101 has beencoupled to the lead screw 113 for a series of successive charactersand'that immediately after the printing of one character a backspacesignal is received. In normal operation the coupling between the printwheel carrier 101 and the lead screw 113 will be maintained as long asthere is a signal indicating that there is information indicative of theidentity of the next alphanumeric character which it is desired toprint. As soon as a backspace signal is detected it is an indicationthat the printing is to be temporarily halted and therefore, soon afterthe last character before the backspacing signal has been printed theletter space roller 151 will be removed from its coupling with the leadscrew 113. The decoupling action occurs in response to a signal from apredetermined timing slot 196 of the timing wheel 195. Immediately afterreceipt of this de-coupling signal the backspace signal will be allowedto actuate the backspace and carrier return solenoid 176. Should thesolenoid 176 be actuated too fast or should the print wheel carrier 101experience an over-travel in the letter spacing direction, there is apossibility that the detent pawl 183 might be pivoted away from thedetent rack 104 and returned so fast that the print wheel carrier 101will not have moved sufficiently far in the backspacing direction toassure a successful backspacing operation. That is, the detent pawl dog184 might engage the same tooth of the rack 104 that it would haveengaged had there been no backspacing operation. From the foregoingdiscussion it can be seen that the rapid withdrawal of the roller 151from the lead screw 113 is important to prevent any overtravel of theprint wheel carrier 101 in the letter spacing direction. As indicated,the rolling contact between the roller 151 and the teeth 167 of the leadscrew 113 facilitates the rapid ejection of the roller 151 and thereforeassures a successful backspacing operation even when the backspacingsignal immediately follows a print signal.

The print wheel carrier 101 may be returned to its left margin positionin response to an actuation of the backspace and carrier return solenoidfor a sufficiently long time to allow the bias force 185 to return theprint wheel carrier 101 to its lefthand position. A means, not shown,may be used to indicate when the print wheel carrier 101 has reached itsleft-hand position and/or to decelerate the print wheel carrier 101during the last portion of its return motion. Other means, not shown,may be employed to uncouple the print wheel carrier 10] from the leadscrew 113 in the event that the print wheel carrier 101 is advanced tothe right beyond a predetermined point.

The print hammer assembly 134 is, in affect, coupled to the print wheelcarrier 101 so that the print hammer assembly 134 is always in aposition to affect the printing of any selected alphanumeric characters.A paper feed mechanism 200 which does not form a part of the presentinvention and which therefore is not described in detail is provided forhandling and indexing the document upon which the printing is beingaffected.

While there has been shown and described what is considered at presentto be the preferred embodiment of the invention, modifications theretowill readily occur to those skilled in the art. It is not desired,therefore, that the invention be limited to the embodiment shown anddescribed, and it is intended to cover in the appended claims all suchmodifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. ln anon the fly printer:

a print wheel having alphanumeric characters and symbols arrangedhelically on the periphery of said print wheel and with a pitch of oneletter space, for sequentially presenting any one of said alphanumericcharacters and symbols at successive letter space positions along aprint line;

a carrier for supporting said print wheel and including means forrotating said print wheel;

' a lead screw having a predetermined pitch along the length thereof andconstantly rotating in time relation with said print wheel;

an engaging mechanism supported on said carrier for selective engagementwith said lead screw;

said engaging mechanism comprising:

a rotatable disc selectively engageable and disengageable with said leadscrew anywhere along the length thereof for advancing said carrier andsaid print wheel from one letter space position to suecessive letterspace positions when said rotatable disc is engaged with said leadscrew; and

control means responsive to the relative angular positions of rotationof said lead screw and said means for rotating said print wheel forselectively moving said rotatable disc into engagement with said leadscrew only when the instantaneous angular position of said lead screwwill result in a noninterferring engagement with the thread of said leadscrew.

2. The combination as set forth in claim 1 wherein said control meanscomprises:

detecting means for detecting the instantaneous angular position of saidlead screw and for generating a command signal when said lead screw isat a predetermined angular position; and

electromagnetic means for urging said disc into engagement with saidlead screw in response to said command signal.

3. The combination as set forth in claim 1 wherein:

said rotatable disc is mounted on a shaft whose axis is at right anglesto the angle of lead of the thread of said lead screw.

4. The combination as set forth in claim 1 wherein:

said control means comprises electromechanical means.

5. The combination set forth in claim 1 further including:

an escapement rack parallel to said lead screw and having teethseparated by a pitch distance which has a fixed relationship with thethread pitch of said lead screw 7 said carrier coupled to saidescapement rack and mounted for reciprocal motion parallel to the axisof said lead screw;

first bias means for urging said carrier to move in' a first direction;

a selectively pivoted escapement pawl coupled to said carrier and forengagement with any of the teeth of said escapement rack to prevent themotion of said carrier in said first direction when said escapement pawlis engaged with any of the teeth of said escapement rack and to enablemovement of said carrier in said first direction when said escapementpawl is disengaged to provide a back space or a carriage returnfunction.

6. The combination as set forth in claim 5 further including:

pawl bias means for urging said escapement pawl towards engagement withthe teeth of said escapement rack; I

electromagnetic means for pivoting said escapement pawl away fromengagement with the teeth of said escapement rack;

said first bias means coacting with said carrier for moving said carrierin said first direction an amount equal to said pitch distance inresponse to the pivoting of said escapement pawl out of engagement withsaid escapement rack by said electromagnetic means followed by thepivoting of said escapement pawl towards engagement with said escapementrack in response to the urging of said pawl bias means when saidelectromagnetic means is de-activated before said carrier has moved anamount equal to said pitch distance.

7. The combination as set forth in claim 5 wherein:

said alphanumeric characters arranged onthe periphery of said printwheel in a helical arrangement are such that when said disc is engagedwith said lead screw to move said carrier, the combined motion of saidcarrier and the rotation of said print wheel causes each of saidalphanumeric characters on said print wheel to be sequentially presentedone time with their approximate midpoints at approximately the samepoint with respect to a referll 12 ence point along the length of saidescapement said electromagnetic means is energized to affect enrack.gagement between said disc and said lead screw 8. The combination as setforth in claim wherein: and including second bias means for urging saidsaid control means comprises electromagnetic disc away from engagementwith said lead screw means. 5 when said electromagnetic means isde-energized.

9. The combination as set forth in claim 8 wherein:

1. In an on the fly printer: a print wheel having alphanumericcharacters and symbols arranged helically on the periphery of said printwheel and with a pitch of one letter space, for sequentially presentingany one of said alphanumeric characters and symbols at successive letterspace positions along a print line; a carrier for supporting said printwheel and including means for rotating said print wheel; a lead screwhaving a predetermined pitch along the length thereof and constantlyrotating in time relation with said print wheel; an engaging mechanismsupported on said carrier for selective engagement with said lead screw;said engaging mechanism comprising: a rotatable disc selectivelyengageable and disengageable with said lead screw anywhere along thelength thereof for advancing said carrier and said print wheel from oneletter space position to successive letter space positions when saidrotatable disc is engaged with said lead screw; and control meansresponsive to the relative angular positions of rotation of said leadscrew and said means for rotating said print wheel for selectivelymoving said rotatable disc into engagemenT with said lead screw onlywhen the instantaneous angular position of said lead screw will resultin a noninterferring engagement with the thread of said lead screw. 2.The combination as set forth in claim 1 wherein said control meanscomprises: detecting means for detecting the instantaneous angularposition of said lead screw and for generating a command signal whensaid lead screw is at a predetermined angular position; andelectromagnetic means for urging said disc into engagement with saidlead screw in response to said command signal.
 3. The combination as setforth in claim 1 wherein: said rotatable disc is mounted on a shaftwhose axis is at right angles to the angle of lead of the thread of saidlead screw.
 4. The combination as set forth in claim 1 wherein: saidcontrol means comprises electromechanical means.
 5. The combination setforth in claim 1 further including: an escapement rack parallel to saidlead screw and having teeth separated by a pitch distance which has afixed relationship with the thread pitch of said lead screw; saidcarrier coupled to said escapement rack and mounted for reciprocalmotion parallel to the axis of said lead screw; first bias means forurging said carrier to move in a first direction; a selectively pivotedescapement pawl coupled to said carrier and for engagement with any ofthe teeth of said escapement rack to prevent the motion of said carrierin said first direction when said escapement pawl is engaged with any ofthe teeth of said escapement rack and to enable movement of said carrierin said first direction when said escapement pawl is disengaged toprovide a back space or a carriage return function.
 6. The combinationas set forth in claim 5 further including: pawl bias means for urgingsaid escapement pawl towards engagement with the teeth of saidescapement rack; electromagnetic means for pivoting said escapement pawlaway from engagement with the teeth of said escapement rack; said firstbias means coacting with said carrier for moving said carrier in saidfirst direction an amount equal to said pitch distance in response tothe pivoting of said escapement pawl out of engagement with saidescapement rack by said electromagnetic means followed by the pivotingof said escapement pawl towards engagement with said escapement rack inresponse to the urging of said pawl bias means when said electromagneticmeans is de-activated before said carrier has moved an amount equal tosaid pitch distance.
 7. The combination as set forth in claim 5 wherein:said alphanumeric characters arranged on the periphery of said printwheel in a helical arrangement are such that when said disc is engagedwith said lead screw to move said carrier, the combined motion of saidcarrier and the rotation of said print wheel causes each of saidalphanumeric characters on said print wheel to be sequentially presentedone time with their approximate midpoints at approximately the samepoint with respect to a reference point along the length of saidescapement rack.
 8. The combination as set forth in claim 5 wherein:said control means comprises electromagnetic means.
 9. The combinationas set forth in claim 8 wherein: said electromagnetic means is energizedto affect engagement between said disc and said lead screw and includingsecond bias means for urging said disc away from engagement with saidlead screw when said electromagnetic means is de-energized.